The present invention concerns an impulse welding device for gas shielded arc welding having a transistor circuitry which is supplied from a D.C. power source, which contains a power transistor and which is controlled by an impulse generator, whereby the welding electrode is supplied with a welding current which changes impulse-wise between a low current level and a high current level.
Electric gas shielded arc welding can be performed using either direct current or pulsating current. Welding with pulsating current has the advantage of a smaller power consumption; however, attention must be paid to the fact that, during the impulse pauses, the current will not be completely shut off but will flow as a spacing or holding current with a low amperage, so that the arc existing between the work piece and electrode will not cease. The current impulses during impulse welding have a repetition frequency between 25 and 100 Hz. The intensity of the current to be connected has a value of from about 600 to 700 A. During the impulse pauses, a spacing current of about 100 A flows. In principle, thyristors can be used as power switches for producing the impulses. However, thyristors which are suitable for circuitry at such high power levels cause switching hesitations by means of which the rectangular impulse wave form is rounded-off. Rectangular impulses with steep impulse trailing edges are needed to maintain the arc. Suitable for generation of these rectangular impulses are transistors which have faster switching behavior than thyristors. A plurality of power transistors connected in parallel are used in the known impulse welding devices. These transistors are synchronously controlled by an impulse source and are alternately switched between a state of a higher conductivity and a state of a lower conductivity. In the lower conductivity state, a spacing current of about 100 A flows through the transistor circuitry and through the arc, whereas in the high conductivity state, the impulse current is, e.g., 700 A. The intensity of this pulsating current is generally determined by the voltage of the D.C. power source, to which the series connection from the transistors and from the arc is connected. At the high current level the transistors are fully conducting, so that there is only a small power loss. At the low current level, on the other hand, the transistors are switched into a condition where they are partially conductive. In this condition, the power transistors are under high voltage, and at the same time a charge current flows through them. This means that the transistors have a relatively high power loss and are heated to considerable temperatures. Therefore, the transistor circuitry of the known impulse welding devices requires extensive water cooling.
The high power loss of the power transistors has the consequence that the known impulse welding devices are designed so as to be very heavy and expensive, and the pulsating component is at all times structurally united with the direct current supply component. In particular, there are no accessory devices which can be operated in connection with direct current welding systems, in order that these direct current welding systems could also be used for impulse welding.
It has been also known to produce an impulse welding current by connecting to the welding circuit a mechanical switch or commutator to which a current limiting resistor is connected in parallel (IEEE Transactions on Industry and General Applications, Vol. IGA-2, No. 3, May/June 1966, pages 225-227).
After all, there has been known an impulse welding device (JA-A-75-19163) which contains in the welding circuit a first series connection of a switching transistor, and an inductivity, as well as in parallel thereto, a second current path of a current limiting resistor and another inductivity. The current transistor is periodically switched to the ON and OFF condition by the pulse generator. The resultant welding current is formed by a train of pulses which are worn off by the inductivities. In particular at the rear slopes of the welding pulses, the formed current intrusions may cause interruptions of the welding arc.